xgf-test.xml (simplistic nonsense example,
just to give you the basic idea)

Some points worth mentioning:

Many functionality is still missing, especially layout information
similar to DG (Diagram) in SGF.

Link concept not final yet (links in comments and from markup there,
but links inside game tree missing)

The structure tries to keep balance between human readability and
machine readability. It therefore makes extensive use of substitution
groups (a technique that's not too common otherwise). The current way of moving
all core markup (e.g. tree structure) into the game specific namespace is also
a result of this. I'm not 100% sure that this is a sensible choice.

As for the file format: I reckon it might be a good idea to
compress XGF files into ZIP files. Not only does this take less
disk space, but also could other related files be
included (e.g. images for players).

Note: Previously on this page there was another attempt of mine to
create an XML file format. It was called SGFML. I keep
it around for historic purposes. It is no longer maintained.